When does ovulation happen?

Approximately 14 days BEFORE the next period — not 14 days AFTER the last one. For a regular 28-day cycle these are the same. For a 30-day cycle ovulation is around day 16; for a 26-day cycle around day 12. The luteal phase (ovulation → period) is roughly fixed at 14 days; the follicular phase varies.

What's the fertile window?

The 6 days ending on ovulation: 5 days before + the day of ovulation. Sperm survive 3-5 days inside the reproductive tract; the egg survives ~24 hours. So intercourse up to 5 days before ovulation can result in pregnancy.

How accurate is this for irregular cycles?

Not very. If your cycles vary by more than 5-7 days, calendar-based prediction misses ovulation regularly. Better tools: ovulation predictor kits (LH surge), basal body temperature tracking, or cervical mucus observation — combined for highest accuracy.

Can I get pregnant outside the fertile window?

Statistically very low (< 1% in any single cycle), but not zero — sperm have been documented to survive up to 7 days, and ovulation timing varies. The fertile window is a probability gradient, not a hard boundary. Don't use this calculator for contraception.

What about PCOS or perimenopause?

Both cause irregular or anovulatory cycles, making calendar-based prediction unreliable. PCOS often skips ovulation entirely; perimenopause causes erratic cycle lengths from 21-60+ days. Consult a doctor or fertility specialist for individualised guidance.

Menstrual Cycle & Fertility Window Calculator

Health & Fitness· 18 tools

Free menstrual cycle calculator. Predict ovulation (typically cycle-day-14 in a 28-day cycle, or cycle_length − 14 in any regular cycle), the 6-day fertile window (5 days before + day of ovulation), and the next expected period. Useful for fertility tracking, family planning, or simply knowing your body. Note: irregular cycles, PCOS, perimenopause, or hormonal birth control change these estimates substantially.

First day of last period

Average cycle length (days)

Typical: 21–35 days. Average: 28.

Formula · Ovulation = first-day + (cycle length − 14)

Fertile window = 5 days before ovulation through ovulation day (6 days total)

Not for contraception · irregular cycles, PCOS, perimenopause make this unreliable

Predicted ovulation

Thu, Apr 30, 2026

Cycle day 14

Fertile window startsSat, Apr 25, 2026

Fertile window endsThu, Apr 30, 2026

Next period expectedThu, May 14, 2026

Disclaimer: For informational purposes only. This is not medical advice — consult a qualified healthcare provider for personal recommendations.

Frequently Asked Questions

References

Related Tools

Frequently Asked Questions

References and sources

Related Tools